1. Field of the Invention
This invention relates to a mechanism for transferring the image on an image-bearing member to a continuous transfer medium, and more particularly to mechanism for transferring an electrostatic latent image or a developed image on an image-bearing member such as photosensitive medium or insulative medium to a continuous transfer medium such as fan fold paper or rolled paper.
2. Description of the Prior Art
An image transfer mechanism for transferring the image on an image-bearing member to a transfer medium will be described with respect to an application thereof to an electrophotographic apparatus using a drum-shaped photo-sensitive medium as the image-bearing member.
Heretofore, electrophotographic apparatuses for recording images by transferring developed images on a photosensitive medium to a transfer medium have sometimes been utilized as the high-speed output device of an information processing system such as electronic computer or the like. In such a case, continuous paper such as fan fold paper or rolled paper has usually been employed as the transfer medium because of its ease of handling and its stability of conveyance. With these apparatuses, in order that developed images intermittently formed on the photosensitive medium in accordance with the presence or absence of recorded information may be transferred to such continuous paper, it is necessary that the transfer paper be intermittently conveyed and the image transfer, namely, the transfer corona charging, take place intermittently in synchronism with the developed images on the photosensitive medium and further that the pressure of an image transfer roller be ON-OFF-controlled to separate the transfer medium from the photosensitive medium to thereby prevent the transfer medium from being stained by fog or the like. However, the method of the prior art has suffered from the following disadvantages which will hereinafter be described by reference to FIG. 1 of the accompanying drawings.
In FIG. 1 which is a schematic cross-sectional view illustrating an example of the apparatus according to the prior art, transfer of the developed image on a photosensitive medium 1 is effected by urging a transfer medium 2 against the photosensitive medium 1 by means of image transfer rollers 4 and 5 (or interior guides) as the transfer medium is conveyed by tractors 6 and 7 having feed pins and being disposed respectively upstream and downstream of the image transfer section, while applying a voltage to the transfer medium by an image transfer corona discharger 3. On the other hand, when the developed image formed on the photosensitive medium in accordance with information is exhausted, the voltage application from the discharger 3 is stopped while, at the same time, the image transfer rollers 4 and 5 are retracted from the photosensitive medium 1. Thereby, the pressure force urging the transfer medium 2 against the photosensitive medium 1 is nulled. At this time, it is necessary to keep the balance between the amount of conveyance by the tractor 6 upstream of the image transfer section and the amount of conveyance by the tractor 7 downstream thereof, to delay the stoppage of the tractor 7 downstream of the image transfer section with respect to the stoppage of the other tractor 6 to thereby prevent the occurrence of wrinking of the transfer medium or unsatisfactory image transfer which would otherwise result from the slack of the transfer medium. However, it is extremely difficult to keep timing between the drives of these two tractors 6 and 7 and an effort to achieve it would result in a complicated construction of the drive control mechanism. In addition to such drive control, the control for the starting of image transfer would be required and these control systems would be very cumbersome. Further, when the expansion or contraction of the transfer medium is taken into account, stable conveyance will be more difficult to do particularly where the image to be transferred is a visible image developed by the liquid development. As a solution to such problem, it would occur to mind, for example, to move each tractor bodily at the image transfer position and effect ON-OFF of the image transfer, but this would involve displacing the drive systems which would unavoidably lead to complication of the apparatus and further to poorer responsivity of the apparatus.